Disorders of perception, attention, and memory frequently accompany the major mental diseases. To understand the neural mechanisms of these mental processes, we are recording the activity of neurons in the extrastriate and prefrontal cortex of monkeys engaged in tasks requiring stimulus discrimination, attention, and memory. When confronted by multiple stimuli, cells in extrastriate cortex appear to engage in competitive interactions, and top-down signals bias these interactions in favor of cells representing the stimulus that is relevant to the task at hand. This bias appears to be accomplished, in part, through high-frequency (gamma) synchronization of the cells carrying the relevant information. Some of the sources of the top-down signals appear to be located in prefrontal and posterior parietal cortex.